On 1/4/2011 1:24 PM, an Arrogant Ignoramus wrote:
what he called
a opinion piece.
I normally do not respond to trolls, but while expressing his opinions,
AI made statements that are factually wrong at least as regards Python
and its practitioners.
1. He correctly notes that the Python Language Reference includes
snippets of BNF grammar with the intention of making the doc clear and
precise.
The if statement is used for conditional execution:
if_stmt ::= "if" expression ":" suite
( "elif" expression ":" suite )*
["else" ":" suite]
He incorrectly claims that the inclusion fails in its purpose. This is
based on the irrelevant fact that 'BNF' has many versions (Python only
uses one, explained in 1.2. Notation) and the false claim that "BNF is
actually not used as a computer language for syntax description.".
Actually, the above snippet is a quotation (with initial ':' expanded to
'::=' and newlines added) from the file Grammar/Grammar:
"if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]"
The CPython program Parser/pgen takes Grammar/Grammar as input and
produces the parser tables that CPython uses to parse Python code. In
other words, CPython uses its well-defined version of extended BNF as a
'computer language for syntax description', contrary to AI's claim. I
presume other implementations make either human or machine use of the
same file.
2. AI also claims that this notation is 'incomprehensible'.
Perhaps to him, but not to me or most the the subset of users who care
about it. One way to expand the BNF into English is as follows:
An if-statement starts with an if-clause consisting of the word 'if', an
expression, a colon, and a suite. 'expression' and a 'suite' have
already been defined elsewhere. The if clause can optionally be followed
any count (including 0) of elif-clauses which are the same as if-clauses
but with 'elif' substituted for 'if'. An if-statement can optionally be
terminated with an else-clause consisting of 'else' and a suite.
Even if such long-winded paraphrases were added to the doc (and I do not
think they should be), in addition to the explanations currently given,
the grammar snippet would still be needed to show the exact technical
definition that CPython actually uses, for those who wan precisely that.
(The people who care include those who want to change the grammar or
those who think the behavior might might match the grammar.)
3. AI's complaint is deceptive and deficient in omitting any mention the
part of the docs *intended* to teach beginners: the Tutorial. The main
doc pages list the Tutorial first, as what one should start with. That
is where I started and I cannot remember if I have ever read the formal
if-statement grammar before, as I knew how to write such before I ever
looked at the Language Reference. Guido and other developers do not and
never have expected people to learn about if-statements from the
grammar. The tutorial says:
"
4.1. if Statements
Perhaps the most well-known statement type is the if statement. For example:
>>> x = int(input("Please enter an integer: "))
Please enter an integer: 42
>>> if x < 0:
... x = 0
... print('Negative changed to zero')
... elif x == 0:
... print('Zero')
... elif x == 1:
... print('Single')
... else:
... print('More')
There can be zero or more elif parts, and the else part is optional. The
keyword ‘elif‘ is short for ‘else if’, and is useful to avoid excessive
indentation. An if ... elif ... elif ... sequence is a substitute for
the switch or case statements found in other languages.
"
I think this says by example and plain English just what a Python
programmer needs to know. It is, of course, followed by many other
examples in the remainder of the tutorial.
If one wants to critique the 'Python Docs', especially as regards to
usefulness to beginners, one must start with the Tutorial; and if one
wants to use if statements as an example, one must start with the above.
--
Terry Jan Reedy
--
http://mail.python.org/mailman/listinfo/python-list
